Mobile communications devices have become an integral part of society over the last two decades. Indeed, more than eighty-two percent of Americans own a mobile communications device, for example, a cell phone. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
Before transmission, the typical mobile communications device modulates digital data onto an analog signal. As will be readily appreciated by the skilled person, there is a plurality of modulations available for most applications. Some particularly advantageous modulations include, for example, continuous phase modulation (CPM). The constant envelope characteristics of this modulation provide for lower energy demands on the power amplifier of mobile communications devices, for example, by reducing the peak-to-average power ratio (PAPR), increasing average transmit power (providing greater transmission range), and increasing amplifier efficiency, i.e. allowing the use of non-linear amplifiers such as Class C amplifiers. Moreover, CPM provides for efficient use of available bandwidth.
A potential drawback of CPM modulations is the use of the inherent memory of the modulation when demodulating/decoding the waveform in order to obtain good demodulator performance. When the mobile communications device receives a transmitted signal which uses a modulation with memory, the decoder uses not only the current signal portion to demodulate but in addition uses information from previous signal portions, i.e. memory, to demodulate the current signal. In other words, the phase of the transmitted signal is dependent on previous signaling intervals.
Decoding modulations with memory increases the computational and memory demands on the transceiver, i.e. a maximum likelihood sequence estimator (MLSE) or the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is typically used to demodulate modulations with memory, thereby increasing the complexity of the device, which may be undesirable in a limited power compact mobile device. More so, when the received signal has a multipath component to it, the size of the MLSE or BCJR trellis structure used to demodulate the signal grows exponentially, which may make practical implementation in a mobile communications device difficult since computational resources are limited.
In typical mobile communications devices that use CPM modulations, the demodulator includes the MLSE and a bank of matched filters. In these approaches, symbol timing, removal of frequency offset, and removal of phase offset may be required for optimum performance. As will be appreciated by those skilled in the art, the removal of phase and frequency offset is required in coherent demodulation, i.e. the phase offset and frequency offset must be known and/or tracked by the receiving device. In non-coherent demodulation applications, i.e. where there is no phase offset and frequency offset tracking information, the performance of the demodulator can degrade quickly with large frequency offsets, for example, the bit error rate (BER) may increase.
In some applications for ultra high frequency (UHF) satellite communications, the device may include a plurality of CPM modems that utilize a fast Fourier transform (FFT) acquisition method to determine the difference in phase (i.e. phase error) and in frequency between the transmit radio and the receive radio (i.e. typically referred to as frequency offset or frequency error). Any residual error in frequency and phase may be reduced with a phase locked loop (PLL). The PLL is typically used in radio communications to create a reproduction of the transmitted carrier frequency (typically with a numerically controlled oscillator (NCO) to generate the signal). This local copy is then used to remove the frequency and phase offset of the received signal. Residual errors in this process may reduce the BER performance of the receiver.
In some applications, the receiver device may have an initial frequency acquisition process—the goal is to estimate the phase and frequency of the incoming signal. A Fourier (or Laplace) transform can be used for known periodic signals while multiple frequency offset hypotheses can be performed for known, pseudo-random sequences. Each hypothesis is generated by rotating the expected pseudo-random sequence by the appropriate frequency offset. The receiver then uses correlation metrics to determine an estimate of the frequency offset.
In some applications, a modem preamble is used that is relatively short and that is designed to allow for non-coherent demodulation of the CPM signal using a simple frequency discrimination method. The preamble comprises an alternating, modulated pattern with +3 and −3 symbols (‘00’ and ‘11’ data values). Due to the nature of some applications, FFT's are difficult to perform at the receiver due to discontinuities in the receive signal stream. In particular, the phase discontinuities appear as sin(x)/x resolution errors in the FFT output. A PLL may require that the alternating pattern (‘11’ and ‘00’) be removed from the received signal to uncover the frequency and phase of the incoming signal. Nonetheless, errors in estimating the value of the incoming symbol and the periodic phase errors due to the nature of the signal reception may increase the time required for the PLL to determine the “lock-in” range (capture) of the received signal and generate the local copy of the transmitted signal with the correct frequency and phase. In a high noise environment, this may prove to be problematic.
One approach is disclosed in U.S. Pat. No. 7,636,399 to Brown et al. The device disclosed includes a non-coherent receiver comprising a bank of CPM waveform matched filters for obtaining branch metrics for each consecutive CPM symbol. The device also includes a recursive inner decoder cooperating with the bank of filters. Although this receiver device is referred to as “non-coherent,” it attempts to estimate the frequency and phase of the signal via the metric computations and a forgetting factor.